1. Field of the Invention
The present invention relates to a semiconductor optical amplifier device, a semiconductor optical amplification driving device and a semiconductor light receiving apparatus, and particularly to a semiconductor optical amplifier device amplifying an externally applied light signal as well as a semiconductor optical amplification driving device and a semiconductor light receiving apparatus.
2. Description of the Background Art
At present, optical communication systems transmit light signals having modulated intensities through optical fibers. The light signal is reflected by an inner wall of the fiber many times when it moves through the fiber, and the signal intensity decreases due to absorption loss and dispersion loss every time the signal is reflected. Consequently, a signal waveform of the light signal deforms, and a signal-to-noise ratio (which will be referred to as an S/N ratio hereinafter) representing the quality of signal deteriorates so that the signal transmission quality lowers. The transmission loss increases with the times of reflection, i.e., with the transmission distance. The increase in transmission loss leads to restriction on the allowed transmission distance.
In addition to factors inside the optical fiber, various factors deteriorate the light signal when it passes through many devices such as repeaters and switches. Deterioration of the light signal becomes a major cause of increasing Bit Error Rate (BER). For preventing the increase in bit error rate, receivers capable of sensing deteriorated weak signals have been developed.
At present, many receivers for the optical communication have employed photodiodes (PDs) using InGaAs semiconductor for a long waveband. By increasing a thickness of a depletion layer of the photodiode, the sensitivity of the receiver can be improved. However, the thick depletion layer of the photodiode deteriorates a high response property, and it is difficult to use such photodiodes in superfast optical communication of a large-capacity in the future.
Recently, Japanese Patent Laying-Open No. 09-275224 has disclosed a new photodiode which is referred to as a Uni-Traveling-Carrier Photodiode (UTC-PD). In a conventional photodiode, holes and electrons form traveling carriers, and holes at a low traveling speed impede fast response. In view of this, UTC-PD does not use holes, and uses only electrons traveling at a high speed so that high response at several hundreds of gigaheltz can be achieved.
However, in the UTC-PD which improves fast response and high output property by using only electrons as traveling carriers, a performance of sensing weak signals is substantially the same as that of the conventional photodiode. Therefore, the UTC-PD is not suitable for light signals which are deteriorated due to transmission loss. Accordingly, such a technique has been developed that overcomes the transmission loss by amplifying the deteriorated signal instead of raising a performance of a receiver.
As a general signal amplifying technique, an optic-electric-optic conversion method has been used. The optic-electric-optic conversion method is a signal amplifying technique in which a light signal is received and converted into an electric signal by a light receiver such as a photodiode, then is amplified and shaped by an electric amplifier, a clock sampling circuit and others, and then is applied to a light source for converting it into a light signal again.
According to advance to high capacity and superspeed of the communication, it has been propelled to actualize all-optic signal processing which does not perform the optic-electric-optic conversion causing a time loss. Some techniques of the all-optic signal processing, which performs signal amplification and waveform shaping on the light signal without conversion, have been proposed.
For example, in a Semiconductor Optical Amplifier (SOA), light is injected into an element having a photo-active layer, and an amplified output light is obtained by causing dielectric emission. Since the SOA basically has the same structure as a semiconductor laser, a production method of the semiconductor laser can be applied to the SOA so that the SOA has an advantage that the structure is small and compact.
However, the method of amplifying the light intensity by the SOA cannot avoid occurrence of noise (ASE noise) due to amplified spontaneous emission. Accordingly, the S/N ratio of the output light deteriorates below a level which allows the use as the signal. Further, there is a lower limit intensity below which the signal cannot be amplified. For example, a quantum cryptography communication, which is expected as an ultimate safe communication method for the future, uses weak signals formed of photons serving as signal units, and therefore cannot use the SOA alone.
Accordingly, such technologies have been developed that overcome a transmission loss by performing waveform shaping on a deteriorated light signal. By way of example, light signal reproduction utilizing a bistable semiconductor laser having a saturable absorption region will now be described with reference to the drawings.
FIG. 17 illustrates an injection light and optical output characteristic curve of a bistable semiconductor laser with a conventional saturable absorption region. In FIG. 17, the abscissa gives light injected into an optical amplification region, and the ordinate gives an optical output intensity obtained depending on the injection light intensity.
As illustrated in FIG. 17, the conventional bistable semiconductor laser exhibits a characteristic of hysteresis in a relationship between the injection light and the optical output. In the following description, a state changing along a path indicated by solid line “A” will be referred to as a lower portion of the hysteresis, and a state changing along a path indicated by broken line “B” will be referred to as an upper portion of the hysteresis.
When the light is supplied into only the optical amplification region of the bistable semiconductor laser, the optical output increases along a path of A from P4 to P1 as illustrated in FIG. 17. In this operation, the light generated in the optical amplification region is absorbed to increase a carrier concentration in the saturable absorption region. Thereby, the optical absorbing effect in the saturable absorption region decreases.
When the light injected into the optical amplification region further increases, the optical absorbing effect becomes saturated. When the injection light intensity attains Pth1, the optical output rapidly increases from P1 to P2. In this specification, this Pth1 is referred to as a rising threshold of the hysteresis.
Conversely, when the light injected into the optical amplification region decreases, the optical output does not rapidly decreases because the saturable absorption region cannot immediately restore the light absorbing effect. Therefore, the optical output slowly decreases through P2 and P3 along a path indicated by B. In this operation, the carrier concentration and the optical output decrease.
When the light injected into the optical amplification region further decreases, the optical absorbing effect in the saturable absorption region is restored owing to decrease in carrier density and optical output. When the injection light intensity attains Pth2, the optical output rapidly decreases from P3 to P4. In this specification, this Pth2 is referred to as a falling threshold of the hysteresis.
The foregoing shape of the hysteresis is affected by the light injection or current injection into the saturable absorption region of the bistable semiconductor laser. When light is injected into the saturable absorption region, photons occur. When a current is injected into the saturable absorption region, photons occur as a result of injection of carriers. Consequently, the photons in the saturable absorption region increase. Thereby, the optical absorbing effect in the saturable absorption region decreases, and the whole hysteresis moves toward a lower side of an injected current value. Further, variations in quantity of the injection light or current cause variations in shape of the hysteresis.
By the light injection or current injection into the optical amplification region, a bias may be applied to the bistable semiconductor laser. When the light is injected into the optical amplification region, carriers occur as a result of injection of photons. Carriers also occur when the current is injected into the optical amplification region. Consequently, the carriers in the optical amplification region increase, and the bias is applied to the bistable semiconductor laser. Further, the bias quantity varies with variations in injection quantity of the light or current.
FIG. 18 illustrates operation characteristics of a conventional light signal reproduction device which uses a bistable semiconductor laser having a saturable absorption region. In FIG. 18, (a) indicates an injection light and optical output characteristic curve illustrated in FIG. 17, (b) indicates a light signal PIN injected into a bistable semiconductor laser having characteristics of (a), and (c) represents output light POUT obtained by injecting light signal PIN at (b) into the bistable semiconductor laser having the characteristics at (a).
Light signal PIN, which is modulated between a light intensity equal to or lower than threshold Pth2 and a light intensity equal to or larger than oscillation threshold Pth1 as illustrated in FIG. 18(b), is injected into the bistable semiconductor laser having the injection light and optical output characteristic illustrated at (a) in FIG. 18. Thereby, the bistable semiconductor laser provides output light POUT amplified as illustrated at (c) in FIG. 18. Japanese Patent Gazette No. 05-507156 has disclosed an example of a light signal reproduction device which performs optical amplification utilizing the bistable semiconductor laser. This will now be described with reference to the drawings.
FIG. 19 illustrates a manner of producing the light injected into the semiconductor laser in the bistable state. In FIG. 19, (a) illustrates deteriorated binary light signal PS, (b) illustrates a clock pulse light PCK applied to (a), and (c) illustrates injection light P1 produced by applying clock pulse light PCK at (b) to light signal PS at (a).
Binary light signal PS at (a) in FIG. 19 has a low intensity equal to or lower than oscillation threshold Pth1 of the bistable semiconductor laser even when it is at a high level. Clock pulse light PCK at (b) in FIG. 19 is produced such that its phase is completely synchronized with that of light signal PS at (a) in FIG. 19. Clock pulse light PCK at (b) in FIG. 19 is applied to injection light P1 at (c) in FIG. 19, and thereby injection light P1 has an amplitude to which an amplitude corresponding to application of clock pulse light PCK is added. Therefore, injection light P1 at (c) in FIG. 19 can vary between Pth1 and Pth2 according to binary values of light signal PS.
FIG. 20 illustrates a structure of a light signal reproduction device 100 performing optical amplification with a bistable semiconductor laser 121.
Referring to FIG. 20, bistable semiconductor laser 121 includes an optical amplification region 122, a saturable absorption region 123 and an active layer 126. Light signal PS illustrated at (a) in FIG. 19 and clock pulse light PCK illustrated at (b) in FIG. 19 are injected through active layer 126 into bistable semiconductor laser 121 which is kept in a state that a constant bias current IB (IB<oscillation current threshold value of bistable semiconductor laser 121) is injected thereinto.
Thereby, light signal PS receiving clock pulse light PCK varies above and below oscillation threshold Pth1 of bistable semiconductor laser 121 so that light signal reproduction device 100 can obtain output light PO having a large amplitude. Thereby, even when light signal PS has a low intensity, which cannot substantially exceeds oscillation threshold Pth1, light signal PS can move to an upper portion of the hysteresis owing to application of clock pulse light PCK. Consequently, amplified output light PO can be obtained.
Though the semiconductor laser in the bistable state can amplify a weak light signal nearly equal to the oscillation threshold, there is a restriction that the light signal can be amplified only if the threshold of the intensity of injection light varies between opposite sides of the threshold. Therefore, the optical amplification by the bistable semiconductor laser cannot be used for the light signal which has a small amplitude or an intensity attenuated below the threshold.
In a method of applying the clock signal to the attenuated light signal having the intensity lower than the threshold, and thereby varying the intensity between opposite sides of the threshold, it is necessary to synchronize completely the phase of the light signal with the phase of the clock signal. In superfast communication, it is generally difficult to adjust the signal for synchronization.